Japanese Patent Laid-Open Publication No. 2001-244528 discloses an optical fiber communication that combines optical signals for data communication on, for instance, Internet, with optical signals for picture communication on, for instance, CATV (Cable Television), by WDM method (Wavelength-Division Multiplexing) for the purpose of effective use of an optical fiber in FTTH (Fiber To The Home) and others.
Data communication on Internet is commonly done with the use of optical signals on direct digital modulation (e.g. “1” for light-on and “0” for light-off (vice versa)) at a data rate such as 622 Mbps and 1.25 Gbps. The optical signals for data communication are formed by successive time-series data composed of packet signals representing communication data such as telegraphic message and idle signals embedded in between the packet signals. That is, as shown in FIG. 1, the communication data such as telegraphic message is transmitted in the form of packets (i.e. packet signals P1, P2, . . . ) intermittently. However, since respective intervals between the packet signals are filled up with idle signals A101, A102, A103, . . . representing “absence of a packet signal”, the optical signals for data communication are transmitted continuously.
An idle signal is commonly provided with a cyclic signal pattern. For instance, according to 1000BASE.X (so-called “gigabit Ethernet” (registered trademark)) defined in ISO/IEC8802.3, the idle signal has a cycle of 20 bits represented as                [00111110101001000101].        
FIG. 2 shows this idle signal in the form of a modulation waveform of optical signals.
In Internet applications, such as website browsing, download of files and e-mail receiving, a downward communication request from a provider to a user discretely occurs in temporal direction. Consequently, in data communication on Internet, much of communication time is occupied with idle signals. Note here that in the data communication on Internet a direction from a provider's side to a user's side will be referred to as “downward direction” and additionally, the reverse direction from a user's side to a provider's side will be referred to as “upward direction”.
Meanwhile, in picture communication on CATV, it is necessary to transmit picture signals of about 100 channels in their multiplexed condition. The current transmission method in popular use is represented by WDM method where picture signals in multichannel are divided with respect to each specified frequency. This is a method where picture signals at individual channels are modulated in analog and thereupon, the modulated picture signals of each channel are arranged with respect to each constant frequency range in sequence. Of course, frequency components of the picture signals at a certain channel concentrate on a frequency range allocated to the same channel.